I have the following problem: I want to install Ubuntu, having already installed Windows 7 and the HD already partitioned. I do not want to erase my data.
During the installation procedure I see the following partitions on my HD:

sda1 - 100 MB

sda2 - 190 GB (this is my windows C:)

sda5 - 285 GB (this is my D: windows partition with my data)

sda4 - 25MB

I guess I should resize the space of sda5 in order to leave nough space for ubuntu and swap memory on sda4. However, will this delete all my files on D:?
I have also no understanding of the 'mounting directory' option (/windows or /home): what is it for?

Ubuntu installation will then find the unallocated space and create sda6 (/) and sda7 (Swap) there during a "side by side" installation.

Ubuntu (and Linux in general) do not use C:, D: etc. to identify partitions. Partitions are mounted at various mount points that can be a folder or a directory. An important mount point is root denoted by /. This is the rough equivalent of C:\ where the OS will get installed. Other partitions are mounted to folders within /. By default /home is a folder in / in Ubuntu. However, one can devote a separate partition to /home by mounting the partition, say /dev/sda8, to that folder. In that case all sub-folders of /home will be in the partition dedicated to it.

Note: the Swap partition has its own format and no mount point, as it is not accessed by the user directly.

Start by defragmenting your drives in Windows. This means that all your Windows files will be in a right order and, more importantly, that your files will be all in the beginning of the drive. The result will be that the end of the partitions will have no more data.

Make a backup of your data anyway. You probably will not loose anything. But it is "good practice".

Partition your D:\ drive. That is to say, divide it into two. The first part will stay D:\ with your Windows data. The other part will host Ubuntu, let's call it "sda6" from now on, as I think it will be named this way.

Increase the size of sda4, since the swap partition should be at least as big as your RAM (memory) capacity.

Install Ubuntu. During the installation, select the option of selecting the partition on which you want it to install ; you will chose sda6, mounted as "/". Format sda4 as a swap partition.

Tip: once Ubuntu is installed, you can still have access to your Windows drives and their content, from Ubuntu.